remove composer.lock, test on travis with lowest deps

Author: keradus

.travis.yml

@@ -9,13 +9,15 @@ php:
   - hhvm
 
 matrix:
+    include:
+        - php: 5.3.3
+          env: 'COMPOSER_FLAGS="--prefer-stable --prefer-lowest"'
     allow_failures:
         - php: hhvm
 
-before_script:
+install:
   - sudo apt-get -y install pypy python-sphinx graphviz
-  - composer selfupdate
-  - composer install --prefer-source
+  - travis_retry composer update ${COMPOSER_FLAGS} --no-interaction
 
 script:
   - cd docs && make linkcheck && cd ..